Semiconductor lasers are well known. High-frequency modulation of the output radiation of semiconductor laser is an active area of optoelectronics. At present, amplitude modulation (AM) is the most widely used scheme. The conventional method of modulating the output amplitude of a semiconductor laser involves varying the laser pumping rate, by varying either electrical pump current or pump photon flux. This method is simple but is known to be limited to relatively low frequencies, typically .ltorsim.10 GHz. Furthermore, the modulation frequencies .gtorsim.1 GHz, the conventional modulation method is plagued by oscillations in the wavelength of the dominant mode of the output radiation. This phenomenon is generally referred to as "chirp". Both of the above referred to shortcomings of the conventional modulation method are due to an intrinsic resonance in the nonlinear laser system, the electron-photon resonance.
An alternative method for modulating the laser output is to directly control by external means the gain coefficient associated with the laser cavity. See, for instance, U.S. Pat. No. 5,023,878, which discloses a semiconductor laser which comprises, in addition to a "gain" section, a "loss" section that is optically coupled to the gain section but is electrically substantially isolated therefrom, such that the modal gain of the laser cavity can be changed through change of the electrical bias on the loss section.
Recently, a different and novel method of varying the gain coefficient associated with the active medium was disclosed. The method involves varying the effective carrier temperature T.sub.e in the laser active region. V. B. Gorfinkel et al., International Journal of Millimeter and Infrared Waves, Vol. 12, p. 649 (1991) disclose heating the electrons in the active region by driving an electric current through the active region, and V. B. Gorfinkel et al., Applied Physics Letters, Vol. 60, p. 3141 (1992) (incorporated hereby by reference ) disclose heating by inducing intersubband absorption in quantum wells in the active region. See also U.S. patent application Ser. No. 07/814,745, filed Dec. 24, 1992 for V. B. Gorfinkel et al. (also incorporated by reference), which inter alia discloses an optical modulator that utilizes carrier heating by means of intersubband absorption. High frequency modulation of T.sub.e by several tens of degrees has been demonstrated experimentally.
Although the method of varying T.sub.e in principle allows faster laser modulation than the conventional (pump current modulation) method, it neither eliminates the relaxation oscillations nor the frequency chirp.
Although at present most laser modulation is AM, there is a growing demand for frequency-modulated (FM) laser output. Coherent optical communication methods based on FM signals are advantageous because varying the optical frequency within the laser amplification bandwidth (approximately 10 nm) opens a larger number of communication channels that are available with AM methods. Existing FM techniques are typically based on the modulation of the optical cavity length of a single-mode laser and can be classified in two groups: those which use the electro-optic effect for the modulation and those that modulate the carrier concentration in specially designed cavity sections. In both schemes, it is the real part of the refractive index which is modulated by the external control means, leading to a variation of the optical path length.
The electro-optic effect typically is very fast. However, as usually implemented in the prior art, electro-optic FM is necessarily accompanied by oscillation in the carrier concentration in the laser, which limits the possible rate of modulation and typically results in output that exhibits amplitude as well as frequency modulation (AFM).
FM schemes that modulate the carrier concentration also exhibit drawbacks. If the carrier modulation occurs in a region of the laser wherein the semiconductor material has a larger bandgap than the material in the active region of the laser then the modulation speed typically is limited by a relatively slow (of order 1 ns) spontaneous recombination. If, on the other hand, the bandgap is the same in the two regions then the recombination can be faster, helped by the stimulated emission process, but the optical output of the laser typically will exhibit AFM.
To summarize, conventional methods of laser modulation, whether they are nominally AM or FM, at high modulation rates (above about 1 GHz) typically result in an unwelcome mixture of the amplitude and frequency modulation. Moreover, the most widely used conventional laser modulation methods, based on the modulation of laser pump rate, are limited to relatively low frequencies (.ltorsim.10 GHz).
It would be highly desirable to have available a method of modulating a semiconductor laser that can make possible higher modulation rates than are attainable with the conventional (pump current modulation) methods, and/or that makes possible substantially pure amplitude-modulated (AM) or pure frequency-modulated (FM) laser output at modulation frequencies higher than the currently attainable maximum frequency of about 1 GHz. This application discloses such a method, as well as apparatus for the practice of the method.
Higher modulation frequencies, and/or substantially pure AM or FM laser output modulation at rates in excess of the currently attainable maximum rate of about 1 GHz, would be of interest in many areas of technology, exemplarily in optical fiber communications and in optical data processing. We anticipate that the below disclosed method and apparatus can be advantageously used in these and other areas of technology.